Electrolytic capacitor and electrolyte therefor



Sept. 19, 1961 D. MOHLER ET AL ELECTROLYTIC CAPACITOR AND ELECTROLYTE THEREFOR Filed Dec. 19, 1958 United States Patent 3,001,108 ELECTROLYTIC CAPACITOR AND ELECTROLYTE THEREFOR Donald Mohler, Saratoga Springs, anrl'Norman W. Cron quist, Glens Falls, N.Y., assignors to General Electric Company, a corporation of New York Filed Dec. 19, 1958, Ser. No; 781,704 8 Claims. (Cl. 317 230) The present invention relates to electrolytic capacitors,

heretofore been considered a practical upper limit for electrolyte systems of water-organic type. In recent years, however, the development of electrical equipment designed for use in increasingly higher ambienttemperatures has placed considerable emphasis on theneed for-electrical components such as capacitors which are capable of reliable operation even under elevated temperature and high voltage conditions.

It is an object of the present invention to provide a capacitor having a high degree of stability and improved life characteristics when operated at high voltages and over a 'wide range of temperature, and in particular is capable of operation up to 150 C. and above.

It is another object of the invention to provide a nonaqueous fill electrolyte which confers the above improved characteristics on electrolytic capacitors.

Patented Sept. 19, 1961 ice of suitable insulating material which serve to seal ofi the opposite ends of casing 10.

In accordance with the present invention, the above objects, and others which will become apparent hereinafter, are achieved in an electrolytic capacitor which comprises a plurality of electrodes at least one of which is composed of a film-forming metal, and an electrolyte comprising diethyl cyanamide and an ionogen, such as potassium thiocyanate, dissolved therein.

The invention will be better understood from the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 shows an electrolytic capacitor roll of a type to which the present invention is applicable; and

FIG. 2 shows a capacitor assembly with the roll body of FIG. 1 enclosed in a casing.

Referring now to the drawing, there is shown in-FIG. 1 a partially unrolled capacitor roll body having electrode foils 1 and'Z, at least one of which is made of tantalum or other suitable film-forming metal and being advantageously, although not necessarily, etched and having a dielectric oxide film formed thereon in accordance with known practice. Spacer strips 3, 4, 5 and 6 composed of porous dielectric material suchas cellulosepaper ofa high degree of porosity are arranged separatingelectrode foils 1 and 2, and the electrodes and spacers are all wound into a compact roll 7 suitable for impregnation with the elec trolyte of the present invention. The roll may be impregnated with the electrolyte either before or after insertion into a casing or other container, as is well known in the art. Tap straps 8 and -9 of opposite polarity are respectively fixed in contact with the electrodes 1 and 2 and serve as capacitor terminals.

FIG. 2. shows a capacitor in, which the rolled capacitor The electrolyte described'herein when adjusted to suitably low resistivities would also be satisfactory for use in sintered slug type tantalum capacitors.

The electrolyte of the present inventionconsists princi-- pally of diethyl cyanamide and a suitable ionogen dis solved therein to provide for the necessary conductivity 1]].

the electrolyte. Generally, the ionogen will be present in, extremely minute amounts, and ordinarily'an amount of the io'nogen is used which will not precipitate at C. Within such limitation, higher concentrations of ionogen may be used for low voltage applications, whereas lower concentrations of i'onogen may be used for higher voltage applications. In general, the range of ionogen content will be from about .01% (or trace amounts) to about 3%.

by weight of the entire electrolyte composition.

A particularly preferred ionogen for the present electrolyte is potassium thiocyanate, this material being readily soluble in the diethyl cyanamide and imparting a desirable flexibility in the particular resistivity which it is desired to obtain in the electrolyte solution. Table I below shows res-istivities which are obtainable with the combination of diethyl cyanamide and potasium thiocyanate, the amount of the latter being shown in percent by weightz l TABLE I Diethyl cyanamide plus KSCN Percent KSON Tempera- Resistivity,

ture, C. Ohm-cm.

25' 380' L0 -55 ag i g Ammonium thiocyanarte is also similarly soluble in diethyl cyanamide and may be used instead of potassium thiocyanate to obtain somewhat similar resistivi-ues as shown above. Other salts which are soluble in diethyl cyanamide may be used, such as alkaline borates, alkyl phosphates, oxalates, citrates, tartrates, succinates, nitrates,

halides, dichromates, and acetates having good solubility characteristics in the diethyl cyanamide solvent of the a present invention.

An electrolyte which has been found particularly satisfactory for use in the present invention hasthe following composition in percent by weight:

ELECTROLYTE vA Percent Potassium thiocyanate 0.02 Diethyl cyanamide 99.98

Life tests made on electrolytic capacitors incorporating.

period of over 2,520 hours. After this period, .theaver age initial room temperature capacitance still retained I in those units was about 85% as measured at 1,000 per second and about 95% at 120 cycles per second.

I The life tests also showed that the present units have excellent insulation resistance even atv 150 C. The units also showed relatively small 'changein powerfactor characteristics even after 2,000'=hours of operation under the aboveconditions s .1 ,s i s t The present electrolyte composition.incorporating diethyl cyanarnide otters ;a number of advantages over electrolytes containing dimethyl forrnamide which has previously been suggested for high temperature applica tions}; The diethyl cyanarnide, for one thing, has a higher a boiling point andis vtherefore less,;volati;1e;at the higher temperature ranges. Of particular,significance-is the slower" vapor pressu-re of diethyl cyanarnide over the entire range of -55 to 150 C. for which the present capacitors, are operable. V i s 'Ilhe physical characteristics of diethyl cyanarnide rep resent a considerable improvement over those of dimethyl By virtue of the above des'cribed characteristics, the

lower freezing point and higher boiling point than conventional fill electrolytes, is not corrosive to, the metals usually employed as capacitor casings, and has no solvent;

action on the formed oxide films of the capacitor electrodes.

peratures, and thischaracteristic provides minimum resistivity change with temperature, as well as minimum capacitance change with'temperature; r M

Being non-aqueous,fthe' present electrolyte has lower vaporpressure than aqueous types of electrolytes and thereby presents a lesser problemin maintaining a tight seal around the capacitor; Moreover, the electrolyte avoids the difliculties encountered in the use of known electrolytes containing mixed. solvents, wherein one of the solvents more readily vaporizes iand thus changes the chemical composition, viscosity, resistivity and freezing point of the electrolyte, and leads to wide changes in capacitance.

There-is thus provided by the invention an electroe lytic capacitor which enables the use of single section single-cased high voltage units having an operatingtem:

-' perature of 150 C. and above. The invention-also present diethyl cyanarnide capacitors may be operated at a highertemperature than dimethyl formamide capacitors with-lower-internal pressure being generated than in the ease' of the latter. Moreover, longer life at any temperature within the operating range will beobtained by I virtue of the lower vapor pressure of diethyl cyanarnide since the physical and chemical characteristics of the electrolyte are less subject to change by leakage or evaporation throughthe seal :A series of comparative tests to determinerespective thermal and chemical stability was ,made between the diethyl cyanamide plus 0.5% of lithium chloride and dimethyl formamidewith 0.5% of lithium chloride were volume reduction of 40-50%.

The following are additional examples of electrolyte compositions which are suitable for use in accordance with the invention, it being understood that the invention is not limited to the particularionogen materials or percentages thereof as set 'forth:

1,) Diethyl cyanarnide plus '1% tetramethylammonium bromide (2) Diethyl cyanarnide plus 1%boric acid (3) ,Diethyl cyanarnide plus 1% ammonium oxalate (4) Diethyl cyanarnide plus 1 lithiumchloride Organic and inorganic salts other than those mentioned above may also be used provided they are not cathodically reduced to 'a metallic or other interfering material at the cathode, or oxidized to an interfering compound at the anode.

s The present diethyl cyanarnide'electrolyte is except'ion'ally'j'stable andresistant under severe conditions of nest, light and chemical change, a has considerably l forming metal, and an electrolyt-e'consistingessentially j b two lekctmlytes, dilathyls cyanarnide and dimethylv of diethyl cyanarnide and an amount of an ionogen di 7 formamide, each having dissolved in it a small amounts for? lithium dichloride as the ionogen. Thesolutionsof makes possible a considerable simplification in themanu v factu re of capacitors while at the same time providing units having improved stability under higher amb ent conditions than heretofore 'considered'permissible;

While the present invention has been described with ref'erencefto particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the artswithout actually departing "from the scope of the invention.v Therefore, the appended claims are intended to cover all suchequivalentvariations as come within the true spirit and scope of the -invention. a a r v V s 7 What we claim ,as new and desire to secure by Letters Patent of the United States is: s I l 1. An electrolytie capacitor co-rnprisingia plurality of electrodes at least one, of which is composed of. a film-- solved xthereiiivwhich will remain in solution at about 55C; V, e .7

2. An electrolytic capacitor comprising a pair of electrodes at least one of which is composed of a film-forming metal, and an electrolyte consisting'e's'sentially, of diethyl cyanarnide and an io'nogen comprising'a thiocyanate compound in sufficient amount to pnovidee'lectrical conducanate in suflicient amount to'provide electrical conductivity in the electrolyte.

4[ An electrolytic capacitor comprisinga pair of clerktrodes at least one of which i-scomposed of a filmforrjning metal, and an electrolyte consisting essentially inpercent by weight of about 99.99%-97% of diethyl cyanarnide and .01-"3% of potassium thiocyanate,

5. An electrolytic capacitor comprising a plurality of electrodes at least one of which is composed of a fil'nn' forming metal, dielectric spacer means betweensa'id elec trodes, and an electrolyte impregnating saiddielectric spacer means,'said electrolyte being composed of a solution of .013% by weightjof potassium thiocyanate in a' solvent com-posed of diethyl cyanarnide.

6. A liquid electrolyte for electrolytic capacitors consisting essentially of an ionogen in a solvent comprising diethyl cyanarnide, the 'ionogen being present in sufiicient amount topr'ovid'e electricalconductivityin the electrolyte; v 7. A liquid electrolyte for electrolytic capacitors ooh- The diethyl cyanamideelectrolyte further has very little change in viscosity with widely varying tern- With the present electrolyte,containing"- practically 100% diethylcyanamideyas much as 50% of the electrolyte could be 'lost without appreciably'ch-ang-f ing the electricalproperties or 'operability of the capacitor;

sisting essentially of a solution of potassium thiocyanate in a solvent consisting of diethyl cyanamide, the potassium thiocyanate being present in sufficient amount to provide electrical conductivity in the electrolyte.

8. A liquid electrolyte for electrolytic capacitors consisting of a solution of Ill-3% by weight of potassium thiocyanate in a solvent composed of diethyl cyanamide.

References Cited in the file of this patent UNITED STATES PATENTS Clark July 4, 1939 Ross Mar. 19, 1957 Schaeren Sept. 9, 1958 Ross Dec. 23, 1958 a Patent New; sgool -loe v UNITED STATESPATENTOFFICE CERTIFICATE OF CORRECTION Septemberl9 1961 fl d' ohl er et all I I It 'ie hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read as corrected below calum 2; lin e vm1, for ."lift" read m life columh i linel for Pl OOO'per", read 1 000 cycles per 5 same column 3 line 46 for "dichloride" read chloride .Signedjand-s ealedthie3rd day of April'l962;

{(SEAL) Attest:

ERNEST W, SW'YIDER Attesting ome a DAVID-L. LADD Commissioner .of Patents Patent No: 3 00l q 108 UNITED STATES PATENT. OFFICE "CERTIFICATE OF coRREcTloNi September 19 1961 Donald Mohl-er etalo It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 6% for "lift" read life 5 column 3 line l for l OOO per" read 1 000 cycles per same column S line 46 for "dichloride" read chloride Signed and sealed this 3rd day of April 1962,

(SEA-L) Attest:

ERNEST W. SWIDER DAVID L ADD Attesting Ufficer Commissioner of Patents 

4. AN ELECTROLYTIC CAPACITOR COMPRISING A PAIR OF ELECTRODES AT LEAST ONE OF WHICH IS COMPOSED OF A FILM-FORMING METAL, AND AN ELECTROLYTE CONSISTING ESSENTIALLY IN PERCENT BY WEIGHT OF ABOUT 99.99%-97% OF DIETHYL CYANAMIDE AND .01-3% OF POTASSIUM THIOCYANATE. 